The objectives of this research encompass: 1. Examination of the chemistry of vitamin B12 and comparison with other low spin cobalt (II) complexes (e.g., of dioximes, Schiff's bases, etc.) of interest as vitamin B12 model systems, including reactions (for example with organic halides) leading to alkylation of the cobalt. At this stage, it is considered important to examine the differences among vitamin B12 and its various model systems, as well as similarities. 2. Synthesis and characterization of new dioxygen adducts of cobalt complexes related to vitamin B12, notably binuclear cobalt complexes in which a pair of bridged cobalt (II) ions act collectively as two-electron donor unit. These studies are of interest, not only in the context of vitamin B12-related chemistry, but also of other biochemical phenomena, for example oxygen-binding and redox processes involving copper enzymes. 3. Examination of the redox chemistry of organocorrinoids and related organocobalt complexes. These studies are directed in part at exploring one mode of cobalt-carbon bond cleavage of possible relevance to coenzyme B12-promoted reactions, as well as to generating and characterizing new cobalt (IV) complexes that were discovered in the course of this research. 4. Examination of the transfer of alkyl groups from organocorrinoids and related organocobalt compounds to mercury(II) and other electrophiles. 5. Studies on the homolytic cobalt-carbon bond cleavage in organocorrinoids and related organocobalt compounds from the standpoint of possible relevance to coenzyme B12-dependent rearrangements.